The present application claims priority to Japanese Application No. P11-229046 filed Aug. 13, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display panel in which devices such as a transistor comprising a display area and a peripheral circuit area are formed on a polycrystalline semiconductor film.
2. Description of the Related Art
In recent years, a display device with large size, fine definition and high-image quality display has been sought. Responding to these needs, an active matrix type liquid crystal display device in which a thin-film transistor (TFT) is employed as a switching device for driving liquid crystal is used as a liquid crystal display (LCD).
As TFT""s used in this liquid crystal display, there exists a TFT in which amorphous silicon is used for a channel region comprising an active layer and a TFT in which polycrystalline silicon is used for the channel region. The TFT using the polycrystalline silicon has lower on-state resistance than the TFT using the amorphous silicon and shows high responsivity and driving ability. Thus, this type of TFT is expected to realize large size, fine definition and high-image quality display.
As a method of forming the polycrystalline silicon layer, an excimer laser annealing (ELA) method for irradiating an excimer laser onto the amorphous silicon and a rapid thermal annealing (RTA) method for irradiating ultraviolet rays thereon are well known.
There is an ELA method called as a multi-scan-shot annealing method. In this method, energy beams are irradiated on a region intended for irradiation in line to crystallize the region. However, heretofore it has been pointed out that this method has the following problems (I. Asai, N. Kato, M. Fuse, and H. Hamano, xe2x80x9cA fabrication of homogenious poly-Si TFT""s using excimer laser annealingxe2x80x9d, in Extended Abst. Int. Conf. on Solid State Devices and Materials, Tsukuba, pp.55-57, 1992). First, it is difficult to irradiate uniform energy beams in a scan direction (horizontal direction). Second, since energy beams are so fine it takes time to irradiate on a large area, thereby decreasing throughput. Third, between neighboring irradiation regions there exists an area which is irradiated by energy beams more than once, whereby the crystal grains become non-uniform.
In recent years, however, an ELA method using high-power energy beams, 10 J and over has been developed (C. Prat, M. Stehle, and D. Zahorski, xe2x80x9c1 Hz/15 Jules-excimer-laser development for flat panel display applications.xe2x80x9d, SID""99, To be presented.). Large-area ELA is prominent as the ELA method. The large-area ELA is a method of irradiating energy beams on the whole area at the same time (T. Noguchi, T. Ogawa, Y. Ikeda, xe2x80x9cMethod of forming polycrystalline silicon layer on substrate by large area excimer laser irradiationxe2x80x9d, U.S. Pat. No. 5,529,951, Jun. 25, 1996; T. Noguchi et al., IEEE Trans. On Electron Device, Vol. 43, pp. 1454-1458, 1996). By irradiating high-power energy beams on an amorphous silicon film, a polycrystalline silicon film with larger crystal grains is formed, so that TFT""s with high carrier mobility are formed (K. H. Lee et al., Gigantic crystal grain by excimer laser with long pulse duration of 200 ns and application to TFT, presented in ISPSA-98. Seoul) and TFT""s in which threshold hardly varies are formed (T. Noguchi, xe2x80x9cThe effect of annealing at a high temperature for short time on micro-poly Si TFT (ELA and RTA)xe2x80x9d, IEICE Technical Report, SDM92-171, 1992-03).
On the other hand, since silicon films tend to absorb ultraviolet rays (UV), annealing is performed by irradiating ultraviolet rays on an amorphous silicon film in the RTA method (J. Mehlhaff, and J. Fair, AMLCD ""93, 1993). In this annealing method light sources of ultraviolet rays are lined up and belt-like ultraviolet rays are uniformly irradiated on the irradiation area of the amorphous silicon film. Thus, a polycrystalline silicon film having uniform crystal grains is obtained.
The active matrix type LCD includes a horizontal scan area (horizontal scan circuit; signal electrode driving circuit), a vertical scan area (vertical san circuit; scan electrode driving circuit), and a pixel area. With a monolithic liquid crystal display panel in which the horizontal scan area, vertical scan area, and pixel area are formed on a substrate, it is required that TFT""s with high mobility are formed in the peripheral circuit area, particularly, in the horizontal scan area, and TFT""s with a large area and uniform properties are formed in the pixel area.
However, the aforementioned annealing methods have the following problems, respectively. With the large-area ELA, the area of the amorphous silicon film which can be crystallized by irradiating one shot of pulse energy beams is limited. For instance, when a whole amorphous silicon film of 20 cm by 10 cm is to be crystallized, supposing energy density necessary for crystallization is 200 mJ/cm2, 200xc3x9720xc3x9710=40 J which exceeds performance capabilities of the current laser apparatus. The problem that exists in the large-area ELA is that in order to poly-crystallize the amorphous silicon film of a region of 10 inches or over, necessary energy is insufficient and the load necessary for large-area irradiation on the optical apparatus increases.
Therefore, in order to irradiate high-power energy beams on a substrate to poly-crystallize across a large area, the high-power energy beams need to be multi-shot irradiated while shifting the irradiation area. For example, assuming that crystallization of the amorphous silicon film of 50 cm2 by irradiating one shot of high-power energy beams is possible, in order to crystallize the whole amorphous silicon film of 20 cm by 10 cm, four shots of irradiation of the high-power energy beams ((20xc3x9710)÷50=4) is necessary. Accordingly, since irradiation needs to be performed while shifting the irradiation area, a problem exists such that there are regions irradiated more than once and these regions have non-uniform crystal grains. Moreover, throughput is reduced, thus there exists another problem such that production costs become high.
On the other hand, the RTA can form the polycrystalline silicon film with uniform crystal grains, so that formation of the TFT""s comprising the pixel area is facilitated. However, in this method it is difficult to melt the amorphous silicon film, thereby crystal grains become small. Thus, there exists a problem such that the TFT""s formed in the polycrystalline silicon film have low mobility and the threshold is non-uniform. That is, as described above it has been difficult to manufacture a monolithic liquid crystal display panel of a large area of 10 inches or over.
The invention has been achieved in view of the above problems. It is an object of the invention to provide a method of manufacturing a liquid crystal display panel in which a polycrystalline silicon film with larger crystal grains and a polycrystalline silicon film with uniform crystal grains can be selectively formed in accordance with a pixel area and each area of a peripheral circuit area, and a liquid crystal display panel with a large area and high-image quality is realized.
A method of manufacturing a liquid crystal display panel of the present invention wherein a pixel area and a peripheral circuit area including a horizontal scan area and a vertical scan area are formed in a polycrystalline semiconductor film on a substrate includes a step of forming an amorphous silicon film in regions corresponding to each area on the substrate, a step of poly-crystallizing the region corresponding to at least the horizontal scan area of the peripheral circuit area in the amorphous silicon film by performing annealing by means of irradiation of laser beams and a step of poly-crystallizing the region by performing annealing by means of lamp lights on the region except the region subject to the irradiation of the laser beams in the amorphous semiconductor film.
In a method of manufacturing a liquid crystal display panel of the invention, at least the amorphous silicon film of a region corresponding to the horizontal scan area of the peripheral circuit area becomes a polycrystalline semiconductor film with larger crystal grains by performing annealing by means of irradiation of laser beams. On the other hand, by performing annealing by means of lamplights an amorphous silicon film in the region not including the region subject to the irradiation of laser beams (that is, the pixel area, further including the vertical scan area) becomes a polycrystalline semiconductor film having uniform crystal grains. Thereafter, thin-film transistors with high mobility are formed in the horizontal scan area and the thin-film transistors having uniform properties are formed in the region not including the horizontal scan area.
Other and further objects, features and advantages of the invention will appear more fully from the following description.